1. Field of the Invention
This invention relates to a two component developing apparatus in a printer in which a toner is supplied from a toner storage unit to a developing unit.
In a printing apparatus, such as a copying machine, a printer, or a facsimile machine and the like, a printing operation is conducted in such a manner that a static latent image is first formed on a photoconductor drum by an electrophotographic system, then developed with a developing agent to form a visual image, and then transferred to a common printing sheet.
To develop the static latent image, a developing unit has been widely used. Particularly, a two component developing apparatus has been used in which two components having stable characteristics are used.
In a two component developing apparatus, it is necessary to mix the toner with the carrier, then electrify the toner, form a magnetic brush consisting of the carrier and toner on a magnetic roller and develop the static latent image on the photoconductor drum. Since the toner is consumed in the developing process, it is necessary to supply the toner.
In accordance with the recent requirements in printing techniques, i.e., a high speed printing and various mode printing, it has been required that the toner is supplied appropriately from a low rate of print to a high rate of print.
2. Description of the Related Art
A known two component developing apparatus comprises a toner container, a magnetic roller for conveying a two component developing agent to a photoconductor drum, paddle rollers for mixing the two component developing agent in the container, a blade for controlling a thickness of the developing agent formed as a layer on the magnetic roller, a flow plate for returning the developing agent which is removed from the magnetic roller by the flow plate to the paddle rollers, a toner density detecting sensor for detecting the density of toner in the container.
The above-mentioned known two component developing apparatus has also a toner hopper (toner supplying means) which comprises a toner cartridge containing the toner, and a toner supplying roller (sponge roller) for supplying the toner in the toner cartridge to the container. The sponge roller has a number of cells (holes).
In the toner hopper, the sponge roller is rotated so that the toner which enters into the cells of the sponge roller falls down into the container and a part of the toner is scraped by a case of the toner hopper and supplied into the container.
Such a toner supply is conducted by rotating the sponge roller in accordance with the output of the toner density sensor or rotating the sponge roller at a constant time interval. The sequence of the sponge roller, such as, the number of cells, the revolutional speed, the gaps between the cells, and the like are determined a standard amount of toner consumed at a standard printing rate, such as, in printing letters (for example, a printing rate: 4-5% in a A4 sized printing sheet). Consequently, the toner supply flow or consumption per unit time is constant.
However, the above-mentioned conventionally known two component developing apparatus has the following drawbacks.
(1) Recently, it is required to print at a high printing density (i.e., rate of print) in, for example, image printing. Therefore, if a printing operation at a high print density (for example, 70-80%) is continued, the toner consumption per a unit time will be increased. Thus, the toner is not supplied enough to the container and therefore the toner will be sufficient, so that the print density will be reduced.
(2) On the contrary, if the toner supply flow per unit time is increased, when the rate of print is low, the toner supply flow will be too much so that the non-electrified toner will increase due to the excess toner supply. Thus, a dark print will occur.